1. Field of Invention
The present invention relates to a multi-chip liquid semiconductor package structure and its method of manufacture. More particularly, the present invention relates to a duplicate chip, duplicate conductive wire package structure capable of shrinking overall package volume while increasing package reliability.
2. Description of Related Art
In this information explosion age, electronic products have enjoyed a close relationship with everybody in this society. Following the rapid progress in electronic technologies, electronic products have become lighter, smaller and more portable. In addition, most electronic products can provide highly personalized functions at an affordable price. Market forces are now pushing semiconductor manufacturers towards manufacturing packages having higher device density to volume ratio. One recent advance of package structure design is the introduction of multi-chip package.
FIG. 1 is a schematic cross-sectional diagram of a conventional lead-on-chip (LOC) chip-stack package structure. The LOC chip-stack package structure shown in FIG. 1 is disclosed in U.S. Pat. No. 5,701,031. The conventional lead-on-chip chip-stack package 100 in FIG. 1 has two chips, including a first chip 130 and a second chip 160, enclosed within a package material 102. The active surface 132 of the first chip 130 and the active surface 162 of the second chip 160 are facing each other. The package 100 also has two lead frames, a first lead frame 134 and a second lead frame 164. The first lead frame 134 includes a plurality of inner leads 136 and a plurality of outer leads 138. The second lead frame 164 has a plurality of inner leads 166 and a plurality of joint sections 168. The inner leads 136 and 166 are attached to the first chip 130 and the second chip 160 via adhesive tapes 140 and 170 respectively. Through wire bonding, the inner leads 136 and the inner leads 166 are electrically connected to the first chip 130 and the second chip 160 respectively by metallic wires 142 and 172. The packaging material 102 encloses the first chip 130, the second chip 160, the inner leads 136 and 166, the joint sections 168, the adhesive tapes 140 and 170, and the metallic wires 142 and 172. To form the package 100, the first chip 130 and the second chip 160 are attached to the first lead frame 134 and the second lead frame 164 using the adhesive tapes 140 and 170 respectively. Wire bonding is next carried out using a bonding machine. Ultimately, the inner leads 136 and the inner leads 166 are electrically connected to the first chip 130 and the second chip 160 respectively by metallic wires 142 and 172. The leads of the second lead frame 164 are next aligned with the leads in the first lead frame 134. Using a YAG laser beam, redundant portion of the joint sections 168 are cut away, and at the same time, the joint sections 168 and corresponding contact points on the first lead frame 134 are welded together. The structure is enclosed with plastic in a molding operation. Finally, dam bars (not shown) on the first lead frame 134 are removed following by the bending of external leads 138.
In the above package, the active surfaces of the two chips are facing each other and hence their respective metallic wires are on the same side. Under such circumstances, contact between a wire on one chip with a neighboring wire on another chip is highly probable and may lead to short-circuiting. A means to prevent short-circuiting is to increase chip separation. However, this will increase the overall package volume and packaging cost. Moreover, using YAG laser to joint the second lead frame 164 onto the first lead frame 134 is an expensive undertaking likely to increase production cost.
FIG. 2 is a schematic cross-sectional diagram of another conventional lead-on-chip (LOC) chip-stack package structure. The LOC chip-stack package structure shown in FIG. 1 is disclosed in U.S. Pat. No. 5,804,874. The lead-on-chip chip-stack package 200 in FIG. 2 also has two chips, including a first chip 230 and a second chip 260 embedded within a packaging material 202. The active surface 232 of the first chip 230 and the active surface 262 of the second chip 260 are both facing up. The package 200 has two lead frames, including a first lead frame 234 and a second lead frame 264. The first lead frame 234 has a plurality of inner leads 236 and a plurality of outer leads 238. The second lead frame 264 has a plurality of inner leads 266 and a plurality of joint sections 268. The inner leads 236 and the inner leads 266 are attached to the first chip 230 and the second chip 260 through adhesive tapes 240 and 270 respectively. The inner leads 236 and the inner leads 266 are electrically connected to the bonding pads 244 and 274 on the first chip 230 and the second chip 260 respectively by metallic wires 242 and 272. The packaging material 202 encloses the first chip 230, the second chip 260, the inner leads 236 and 266, the adhesive tapes 240, 270, 276 and 204, and the metallic wires 242 and 272. The purpose of putting additional adhesive tape 204 is to increase distance of separation between the lead 242 and the second chip 260 so that probability of electrical contact between the two is minimized. To form the package 200, the first chip 230 and the second chip 260 are attached to the first lead frame 234 and the second lead frame 264 using the adhesive tapes 240 and 270 respectively. Wire bonding is next carried out using a bonding machine. Ultimately, the inner leads 236 and the inner leads 266 are electrically connected to the bonding pads 244 and 274 on the first chip 230 and the second chip 260 respectively by metallic wires 242 and 272. The leads of the second lead frame 264 are next aligned with the leads of the first lead frame 234. The joint sections 268 of the second lead frame 264 are electrically connected to the first lead frame 234. The structure is enclosed with plastic in a molding operation. Finally, dam bars (not shown) on the first lead frame 234 are removed following by the bending of external leads 238.
In the above package structure, quite a few adhesive tapes are used. Since adhesive tapes have intrinsic tendency to absorb moisture at the processing stage, frequency of delamination of the package may intensify leading to a lower yield and reliability problem. Thus, the product may subsequently be damaged. Furthermore, short-circuiting between the lower metallic wires and the upper chip is prevented by putting up thick adhesive tapes. Ultimately, overall volume of the package will increase leading to an increase in production cost.